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Volume 31, Number 4—April 2025
Research

Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023–2024

Philippe Noriel Q. Pascua1, Anton Chesnokov1, Ha T. Nguyen, Han Di, Juan De La Cruz, Yunho Jang, Andrei A. Ivashchenko, Alexandre V. Ivachtchenko, Erik A. Karlsson, Borann Sar, Chin Savuth, Timothy M. Uyeki, Charles Todd Davis, and Larisa V. GubarevaComments to Author 
Author affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (P.N.Q. Pascua, A. Chesnokov, H.T. Nguyen, H. Di, J.D.L. Cruz, Y. Jang, T.M. Uyeki, C.T. Davis, L.V. Gubareva); ChemDiv, San Diego, California, USA (A.A. Ivashchenko, A.V. Ivachtchenko); Avisa LLC, Hallandale Beach, Florida, USA (A.V. Ivachtchenko); Virology Unit, Institut Pasteur du Cambodge, Phnom Penh, Cambodia (E.A. Karlsson); US Centers for Disease Control and Prevention Country Office, Phnom Penh (B. Sar); National Institute of Public Health, Ministry of Health, Phnom Penh (C. Savuth)

Main Article

Table 2

Susceptibility of highly pathogenic avian influenza A(H5N1) viruses isolated from humans to M2 blockers in cell culture–based IRINA, 2023–2024*

Influenza A(H5N1) virus Differences in M2 protein sequence
 Mean EC50 ±SD, ng/mL
 GISAID ID
12 13 14 18 28 31 50 51 61 82 88 89 95 Aman Riman
Clade 2.3.4.4b
A/bald eagle/FL/2022† K N G N I S C V G S D G E 30.22 ± 13.33 8.36 ± 1.99 EPI_ISL_15063846
A/Chile/25945/2023‡ . . . . . . . . . . . . . 95.43 ± 11.69 16.70 ± 6.71 EPI_ISL_17468386
A/Chile/25945/2023 clone‡ . . . . . . . . . . . . . 26.73 ± 3.24 6.90 ± 0.52 NA
A/Texas/37/2024 . . . . . . . . . . N . . 24.11 ± 5.94 6.57 ± 1.71 EPI_ISL_19027114
A/Michigan/90/2024 . . . . . . . . . . N . . 29.60 ± 4.52 8.06 ± 1.54 EPI_ISL_19162802
A/Colorado/109/2024 . . . . . . . . R . N . . 14.88 ± 3.17 4.22 ± 0.02 EPI_ISL_19263923
A/Colorado/134/2024 . . . . . . . . R . N . . NT NT EPI_ISL_19280426
A/Colorado/137/2024 . . . . . . . . . . N . . 25.21 ± 3.00 6.61 ± 0.57 EPI_ISL_19294963
A/Colorado/138/2024 . . . . . . . . . . N . . NT NT EPI_ISL_19294962
A/Colorado/139/2024 . . . . . . . . . . N . . 17.65 ± 4.39 5.30 ± 0.69 EPI_ISL_19294964
A/Missouri/121/2024 . . . . . . . . . . N . . VNR VNR EPI_ISL_19413343
A/California/134/2024
 .
 .
 .
 .
 .
 .
 .
 .
 .
 .
 N
 .
 .
14.14 ± 3.07
 4.28 ± 0.60
 EPI_ISL_19463619
Clade 2.3.2.1c
A/Cambodia/NPH230032/2023§ R K E R V N Y I R N . S . >1,000 >1,000 EPI_ISL_17024123
A/Cambodia/2302009/2023§ R K E R V N Y I R N . S . >1,000 >1,000 EPI_ISL_17069010
A/Cambodia/NPH230776/2023 . . . . . . . . R . . . . 43.27 ± 10.46 12.27 ± 7.79 EPI_ISL_18373263
A/Cambodia/2310209/2023 . . . . . . . . R . . . . NT NT EPI_ISL_18366401
A/Cambodia/KSH230332/2023 . . . . . . . . R . . . . 41.89 ± 13.31 9.84 ± 2.28 EPI_ISL_18543355
A/Cambodia/2311257/2023 . . . . . . . . R . . . K 59.73 ± 16.49 12.97 ± 1.74 EPI_ISL_18543643
A/Cambodia/24020155/2024 . . . . . . . . R . . . . 32.36 ± 10.13 6.48 ± 1.88 EPI_ISL_19270605
A/Cambodia/24020179/2024 . . . . . . . . R . . . . 34.73 ± 15.95 7.25 ± 2.96 EPI_ISL_19270607
A/Cambodia/SVH240441/2024
 .
 .
 .
 .
 .
 .
 .
 .
 R
 .
 .
 .
 .
NT
 NT
 EPI_ISL_19312044
Control viruses¶
A/Wisconsin/53/2009 (H1N1)pdm09 R S E R . . . I R . . . . 25.49 ± 4.98 6.48 ± 1.46 EPI_ISL_63269
A/California/07/2009 (H1N1)pdm09 R S E R . N . I R . . . . >1,000 >1,000 EPI_ISL_203615

*Data shown are means (+SDs) of >3 experiments. Concentration range of the M2 blockers used was ≈3.9–1,000 ng/mL. Dots indicate same amino acid residue as in A/bald eagle/Florida/W22-134-OP/2022. Amino acid substitutions at residues L26, V27, A30, S31, and G34 were implicated in resistance to M2-blockers. Aman, amantadine; EC50, 50% effective concentration; ID, identification; IRINA, influenza replication inhibition neuraminidase-based assay; NA, not applicable; NT, not tested; riman, rimantadine; VNR, virus not recovered. †Clade 2.3.4.4b influenza A(H5N1) virus, A/bald eagle/Florida/W22–134-OP/2022 (GISAID ID no. EPI_ISL_15063846; https://www.gisaid.org), was used as control. ‡See Appendix 1 Table 2). §Viruses collected from 2 family members in early 2023 that contain the M2-S31N resistance marker (in bold). ¶The M2 protein of the A/bald eagle/Florida/W22–134-OP/2022 differs from the control seasonal influenza A(H1N1)pdm09 virus by an additional 3 amino acid and A(H3N2) virus by an additional 10–11 amino acids.

Main Article

1These first authors contributed equally to this article.

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